Place them both in the same directory. Make sure the subtitle file has the same filename as the video with the .ass extension (if the video is "120703 NMB48 Geinin!.mp4", the subtitle should be "120703 NMB48 Geinin!.ass"). Then just play the video, the player should automatically pick up the subtitle. If not, you need to load them manually (find a "Load subtitle" menu somewhere in the View or Video section of the video player).